We agree that definitions of psychopathology items are not independent of diagnostic concepts and that this is a limitation of the current conceptual framework of psychopathology. It must be noted, however, that although the DSM and ICD classification systems were based largely on expert opinion, with the aim of improving reliability, and were not the outcome of rigorous nosological validity studies, they cannot be considered entirely arbitrary. Indeed, there are studies which provide support for some validity in terms of temporal stability of diagnosis and long-term outcome (Reference Mason, Harrison and CroudaceMason et al, 1997; Reference Amin, Singh and BrewinAmin et al, 1999). In addition, the current widespread use of these two main diagnostic systems and the huge impact they have on psychiatric training make it difficult to use any set of clinical descriptors that are really free from their influence.

The second point raised by Craddock et al concerns the need for independent external validators of psychopathological dimensions. We agree fully with this comment. Our aim is to further the analysis of the dimensions we have identified by examining them against those validators that are currently considered the most objective, such as neuroimaging, genotypic, neuropsychological and neurophysiological data.

Like Craddock et al, we hope that future developments in molecular genetics and neuroscience will provide grater insight into the aetiology of psychiatric disorders. However, we would point out that one of the leading American psychiatric geneticists, Ken Kendler, has recently cautioned against an expectation that genetics will provide definitive answers to the complex and multifaceted problems currently facing psychiatric nosology (Reference KendlerKendler, 2006). Nevertheless, we retain our hope that the analysis of psychopathological dimensions, even if the latter are based on symptoms influenced by the current nosological categories, will help to clarify heterogeneity among patients with psychotic illnesses and facilitate our understanding of the underlying pathophysiological pathways.